1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus, in particular, capable of selecting one of a tomographic mode (B-mode) for obtaining a tomographic image and a Doppler mode (D-mode) for obtaining blood flow data or two-dimensional blood flow image (color flow mapping image).
2. Description of the Related Art
The ultrasonic imaging apparatus is used as an ultrasonic diagnosis apparatus for scanning a subject with an ultrasonic beam to obtain a tomographic image or a blood flow image of the subject, and diagnosing the subject based on the obtained image. In order to scan the subject with the ultrasonic beam, it is necessary to converge or deflect the ultrasonic beam. As means for converging or deflecting the ultrasonic beam, delay circuits are provided in an ultrasonic transmitter circuit and an ultrasonic receiver circuit. In general, an analog delay element, for example, an LC delay element, is used as the delay circuit. However, where the LC delay element is used, it is difficult to attain a long delay, and precision of delay time is not satisfactory. In particular, if the frequency of a drive signal is increased, a sufficient delay performance is not obtainable by the LC delay element.
Under the circumstances, it has been proposed to form the delay circuit and its peripheral section in a digital manner. In this case, much consideration must be given to the use of an A/D converter. An ultrasonic signal has a wide dynamic range from a high signal level to a low signal level. An A/D converter for directly A/D converting the signal of a wide dynamic range is not generally employed and is expensive. If such an expensive A/D converter was provided in each of a great number of channels of an ultrasonic imaging apparatus, the manufacturing cost of the apparatus would increase.